Davis quadrant or Backstaff, 1730
John Dobson, London
Maker:
Collectie:
NavigArte
Although already described in 1595 by John Davis, the backstaff or Davis quadrant only came into widespread use from around 1680, more so in England than in the Netherlands. It is a shadow instrument that allowed the navigator to observe the Sun without looking directly at it, thus avoiding eye damage.
The measured angle on the small 60° arc had to be added to the measured angle on the larger 30° arc.
Backstaff, signed on ivory plaque MADE BY JOHN DOBSON 1730 FOR MR THOMAS SALKELD, with lignum vitae frame, boxwood arcs and replaced vanes, the 30 degree transversal scale with fleur de lys stamped decoration, the back with scales for the Sun's declination and the right ascension, 22 in (56cm) long,
The 1°- 62° scale has a Type A style of the 65° arc: no outer scale for correcting the sun’s semi-diameter, as is found in 32% of instruments examined by N. De Hilster.
This type was only found in European instruments up to 1740